Dogs are the great pet if you are looking for a new best friend. They are sociable and playful and crave attention. This could be a downside to some as they do need to be trained well and given lots of attention.
Cats are perfect for people who want a cuddly pet that will have some of its own independence. Cats are much less needy than dogs but do lack sociability in some cases.
Rodents can be a great pet, especially as a first pet for children. They can teach a child responsibility, but their cage needs constant cleaning to avoid odors.
Reptiles are a perfect pet for someone searching for a low maintenance animal as they are often easy to keep. Reptiles do not often bond with their owners however, so keep that in mind if that is important.